Nowadays, the integration degree in electronic ICs is more and more increased and developed. Heat dissipating has become the most important problem in all the electronic devices. Take portable computer as an example, the heat generated from the high speed processing ICs will critically affect the stability of the system, if heat can't be well dissipated. Therefore, the fan inside the computer is so-called the active dissipating element, which is the most useful and basic requirement for the portable computer. Please refer to FIG. 1. FIG. 1 illustrates a schematic block diagram of the foregoing fan controlled by a pulse width modulated (PWM) signal for heat dissipating. The main elements are the fan motor 10 and the speed output module 11. When the system between the first terminal 101 (power source) and the second terminal 102 (ground potential) of the fan motor 10 provides a PWM signal (as shown in FIG. 2(a)), the fan motor 10 changes its speed in response to the frequency and the duty cycle of the PWM signal, and then the speed output module 11 outputs a square wave signal (as shown in FIG. 2(b)) in response to the speed of the fan motor 10. The frequency of the square wave signal is proportional to the speed. According to the arrangement of the electronic devices illustrated by FIG. 1, the speed output module 11 is powered by the PWM signal. Thereby, the speed output module 11 outputs the square wave signal only when the PWM signal is active. In other words, the speed variation of the fan can't be monitored in the whole process. For the system which changes the duty cycle of the PWM signal corresponding to the fan speed and the ambient temperature, errors might be generated because of the inaccuracy resulted from the unstable monitoring time. That will affect the control of the fan speed in the system, and it will be very difficult to get a balance among the heat dissipating efficiency, energy consumption, and the noise generated from the fan.
From the above description, it is known that the conventional motor speed detecting device is not very practical when the user needs to precisely control the fan speed in the whole process. It is needed in the industry that an apparatus that is designed for detecting the fan speed allows the fan to be precisely controlled by the system in the whole process.